A method and a device for operating an internal combustion engine is referred to in German patent document 44 00 536. According to this method, secondary air is introduced into the exhaust-gas pipe system during operation of the internal combustion engine and cools the high temperature segment of a catalytic converter as well a muffler element. Modern catalytic converters reach temperatures at a high engine load (full load) of approximately 1000° C. If the vehicle is switched off shortly after such full load operation, the outer wall temperature of the catalytic converter may quickly reach several 100° C. This results to a certain extent in a heat build-up since during still stand of the vehicle, there is no cooling from the head wind.
However, stopping the vehicle with a running engine following full load operation is less critical since the comparatively cool exhaust air of the internal combustion engine that is at a temperature of ca. 300° C. to 400° C. during idling is able to draw off excessive heat from the hotter catalytic converter, thereby cooling the catalytic converter via the exhaust air. The heat build-up at vehicle still stand with a switched off engine may heat the catalytic converter wall to the extent that dry grass, for example, on which the vehicle is parked may catch fire. Some automobile manufacturers activate a warning light in the vehicle cockpit given a risk of grass catching fire as a result of the heat radiation from the catalytic converter, which also involves the risk of vehicle fire. The driver may then move the vehicle to a different parking location or allow the internal combustion engine to idle for a while in order for the catalytic converter to cool.
A vehicle also emits pollutants after shutoff. Among others, small quantities of hydrocarbons (HC) escape from the induction pipe into the environment. Sources of such HC emission include, for example, crankcase ventilation or residues on an injection valve or tank ventilation.
These emissions will have to be eliminated due to stricter exhaust-gas limit values, in the future.